Leukemia is induced by radiation in rodents; quite easily in most strains of mice, less readily in rats. The type of leukemia most frequently induced in mice is lyphoid; in the rat, as in man, it is myeloid. The incidence of solid lymphoid tumors in rats is about equal to that of leukemia. The incidence of other types of cancer is very high in the irradiated parabiont rat. This study of parabiont pairs is being carried on in particular reference to: 1) incidence and types of leukemia and lymphoid tumors; 2) determination of whether the inhibiting effect of large doses of radiation of the rat on leukemogenesis is real or apparent; 3) determination of the apparent greater incidence of leukemia in female parabiont rats and of reasons for this; 4) comparison of radiation leukemogenesis in rat and in mice.